A typical automatic transmission (AT) includes a torque converter and a powertrain of a multiple speed gear mechanism connected to the torque converter. In addition, a hydraulic control system may be provided with the AT for selectively operating at least one operational element included in the powertrain, according to a running state of a vehicle.
When it is determined that an upshift is required on the basis of a vehicle speed and throttle valve opening, a transmission control unit (TCU) for controlling an AT starts upshift control by starting control of a solenoid valve in the AT, which is usually called “Shift Start point” and is abbreviated as “SS point”.
By starting of the solenoid valve control, after a certain period, an off-going frictional element begins releasing its hydraulic pressure, and an on-coming element begins to be supplied with a hydraulic pressure, which is usually called “Shift Begin point” and is abbreviated as “SB point”. The period after the SS point to the SB point becomes a delay period which is not used for an actual shifting operation of the AT.
So, an actual shifting period (also called an inertia phase) of the AT begins at the SB point and finishes at a time point at which the off-going element is fully disengaged and the on-coming element is fully engaged. Such a time point at which the off-going element is fully disengaged and the on-coming element is fully engaged is usually called “Shift Finish point” and is abbreviated as “SF point”.
Sometimes, the above mentioned delay period between the SS and SB points is described as a sum of a preparation phase and a torque phase. However, in the specification and the appended claims, such a delay period from the SS point to SB point is integrally referred to simply as a torque phase period.
If the above-mentioned delay period (i.e., a torque phase period) is lengthened, an actual shifting period (i.e., an inertia phase period) may be shortened such that a shift shock results.
Usually the delay period is learned while undergoing a shifting operation of the AT such that a subsequent upshift may be controlled based on the learned delay period. According to the prior art, the learning of such a delay period is not optimized with respect to a running state of an engine, such as with respect to engine torque.
For example, when an engine is run with a high output torque, the torque phase should be lengthened for sufficient shift quality. However, since the torque phase period that is learned may not be as sufficiently long as required, shift quality is deteriorated or a shift shock results.
On the other hand, when an engine is run with a low output torque, the torque phase should be shortened for a rapid response. However, since the torque phase period that is learned may not be as sufficiently short as required, a total period for an upshift may be lengthened more than necessary.
In other words, optimizing the delay period with respect to an engine torque may promise a reduction of shift shock in the case that an engine is run with a high output torque, and reduction of a period consumed for an upshift in the case that the engine is run with a low torque.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art that is already known in this country to a person of ordinary skill in the art.